A history. I've successfully updated M/B / BMC / ME / FRUSDR to the current FW versions (latest pack available), then successfully updated backplane's AXX6DRV3GEXP FW to version 2.18. After it I've stuck trying to update AXX4DRV3GEXP to this version all the time with the same error mentioned above.

After some searching I've found some posts with Aryan2411 advices and tried to follow all of them, I believe, as well as tried to use those versions of fwpiaupd.efi and ipmi.efi from his "Everything is explained clearly.zip" file with no success (error sounds like "not a valid image" for executing those versions). Also I've tried to check the IPMB cables' connection as well as connected the "lovely" backplane to HSBP_A on the M/B alone. The same thing.